Heulandite is a calcium (Ca), sodium (Na), hydrated aluminosilicate mineral with a general chemical formula of (Ca,Na)_2–3Al₃(Al,Si)₂Si₁₃O₃₆•12H₂O. Heulandite is in the zeolite group, a somewhat remarkable set of minerals in that their open, channel–like crystal structure permits continuous (and in part reversible) dehydration and exchange. These characteristics allow zeolites to absorb, hold, release, and exchange a wide range of natural and manmade constituents. In soils, heulandite commonly is inherited from soil parent materials, typically volcanic rocks or tuffaceous sediments or less commonly may form directly in saline–affected volcanic or nonvolcanic soils through pedogenic processes, depending on soil pH and the concentration of silicon (Si), aluminum (Al), sodium (Na), and Ca.

The distribution of mineral resource deposits with zeolites as a commodity (major or minor) in the United States, extracted from the U.S. Geological Survey (USGS) Mineral Resource Data System (MRDS) website, can be seen by hovering the mouse here. Statistics and information on the worldwide supply of, demand for, and flow of zeolites, including heulandite, are available through the U.S. Geological Survey (USGS) National Minerals Information Center (NMIC) website.

In our data, heulandite is exceedingly rare, present at only 56 sites in both the soil A and C horizons (see the summary statistics [open in new window]). Because so few samples have heulandite concentrations above the detection limit of 0.2 weight percent (wt. %), the spatial distribution of heulandite is shown as a proportional symbol map rather than an interpolated and smoothed color surface map.

The majority of heulandite occurrences in soils, such as those in Oregon, Idaho, Montana, Wyoming, Nevada, Utah, Arizona, New Mexico, and Texas, likely have an original source from volcanic rocks. Heulandite in many of these sites can be linked to a local presence of mafic volcanic rocks, tuffaceous sediments, volcaniclastic deposits, or alluvial or eolian deposits derived from volcanic rocks. In southwestern Wyoming, a cluster of heulandite occurrences is present in soils developed on basin–floor sediments, dominated by sandstone, shale, and tuff. Other scattered occurrences in North Dakota and South Dakota have no apparent link to nearby volcanic rocks, although a distant atmospheric volcanic ash source is likely. Typically, where heulandite occurs, it is present in both the soil A and C horizons, suggesting a similar volcanic source for the mineral. At those sites where heulandite was detected in both soil horizons, the concentration in the soil C horizon is somewhat higher than the concentration in the soil A horizon, suggesting possible heulandite dissolution in surface soils because of weathering.